In parallel with rapid popularization of the Internet, the infrastructure for easy data access and dispatch at any place and at any time is being proceeded. At present, the main roll of interface devices is played by a "Personal Digital Assistant" (hereinafter abbreviated PDA).
The PDA, being for mobile use, is desired to be thin, light and having less power consumption. A reflective liquid crystal display device (hereinafter abbreviated LCD) which does not need a back light is considered to be most suitable for this purpose. Thus, the majority of PDA products adopt the reflective LCD.
The PDA is expected to harmonize functionally with the display units of a hand-held computer, a note book type personal computer, a network computer, a digital camera, etc. It is also expected to build a huge market as a mobile business tool which is a main device of "The Mobile Office" which has been proposed as a new business environment in parallel with infrastructure setup such as high speed communication systems, mobile communication operating systems, etc.
Although a black and white reflective LCD is mostly used at present, a color reflective LCD is desired. Research and development are actively being done in this area. Therefore big demand of the color reflective LCD is expected as a key device of the mobile business tool.
Although various types of reflective LCDs are researched and developed, a single polarizer type is proposed as a promising type for realizing a bright display in place of a double-polarizer type. In LCDs, there are two types. One is an active matrix type and another is a single matrix type. The former is for performance oriented use and the latter is for cost oriented use.
The structure of the single polarizer LCD is shown, for example, in the following publications:
1) Regarding the black and white reflective LCD, PA1 2) Regarding the color reflective LCD,
I. Fukada, M. Kitamura, Y. Kotani: ASIA DISPLAY '95 pp. 881-884 (1995). PA2 H. Yamaguchi, S. Fujita, N. Naito, H. Mizuno, T. Otani, T. Sekime, T. Ogawa, N. Wakita: SID 97 DIGEST pp. 647-650 (1997).
FIG. 10 shows a representative example of a conventional single polarizer color LCD. On a substrate 201, striped reflective electrodes 207a-207f are formed. Between the substrate 201 and its facing transparent substrate 208, a liquid crystal layer 206 is sandwiched. For the liquid crystal layer 206, "Twisted Nematic" (hereinafter abbreviated TN) liquid crystal or "Super Twisted Nematic" (hereinafter abbreviated STN) liquid crystal is used. In most cases, the TN liquid crystal is used for the active matrix type and the STN liquid crystal is used for the single matrix type. At the inward-facing surface of the transparent substrate 208, color filters 203a-203f are formed corresponding to reflective electrodes 207a-207f, and also striped transparent electrodes 205 are formed sandwiching protecting layer 204 in-between, and directing the stripes to a right angled direction with that of the stripes of the reflective electrodes 207a-207f. At the outward-facing surface of the transparent substrate, a polarizer 211, a phase compensation plate 210 and a scattering plate 209 are laid bonded from outside in order.
The single-polarizer reflective LCD provides a brighter display compared with the double-polarizer reflective LCD because the light passing frequency through the polarizer(s) decreases from four times to two times. Also, in the single polarizer type, the reflective electrodes 207a-207f can be built in liquid crystal cells, so that a display of less parallax is obtainable. Especially in case of a color reflective LCD which adopts color filters, the single polarizer type can minimize the probability that incoming light and outgoing light pass through different color domains, which also provides a high color purity and brighter display.
For getting good alignment of LCD, the plane surface of an electrode is desired. However, in the plane reflective electrode, due to the strong specular reflection, the reflective luminance except for certain directions becomes extremely small, so that a direct view reflective LCD cannot be realized as it is. In order to cover this problem, a scattering plate 209 is disposed in a light propagation path. In the single-polarizer reflective LCD, different from the double-polarizer reflective LCD which has a scattering reflective plate at the back side, the optical system is so composed that the scattering function is separated from the reflection function.
In most cases, aluminum (hereinafter abbreviated Al) is used for the reflective electrodes 207a-207f because Al is less expensive and has a high reflective characteristic of less wavelength dependence.
However such conventional reflective LCD has the following problems. Since the reflective electrodes 207a-207f are carried through an inspection process and so on exposing the surface, scratching is liable to occur. Thus, the handling is difficult.
Also, since the reflective electrodes 207a-208f are exposed during cell assemble processes, the durability to an acid, alkali or a solvent is not sufficient, so that the corrosion by such chemicals are worried.
Also, since the reflective electrodes 207a-207f contact directly with the liquid crystal, it is difficult to maintain durability against corrosion. Particularly, it is difficult to keep reliability in a constant-voltage loading test under high temperature and high humidity conditions.
The above problems are especially liable to occur when Al is used. The present invention aims to provide, by relatively simple structure, a bright and less parallax reflective LCD which is also superior in processing and reliability.